The Energy Independence and Securities Act of 2007 (EISA) established mandatory Renewable Fuel Standards (RFS) that require an increase from 9 billion gallons in 2008 to a minimum of 36 billion gallons of renewable fuels to be blended in transportation fuels sold in the US by 2022. Today, we face a challenge in meeting these mandates partly because of the energy requirements of generating advanced biofuels as well as sourcing an adequate supply of reliable feedstocks.
To date, the microalgae-to-renewable fuel pathway has focused on the generation of lipids within the microalgae, followed by the extraction and conversion of those lipids to biodiesel. The process relies on the microalgae to generate large concentrations of lipids in order to make the process energy return on investment (EROI) favorable. Since specific high lipid-yielding microalgae are selected for cultivation, contaminants such as other species of algae and predators only lower expected lipid yields. Thus, in practice, lipids yields have consistently been lower than predicted because of these factors.
The man-made process of hydrothermal liquefaction (HTL) of biomass-to-biocrude mimics the natural process of applying heat and pressure to decayed biomass in the earth's crust to form fossil fuel crude oil over the course of millennia. In this process, nearly all of the organic fraction (lipids, carbohydrates, protein, and nucleic acids) of the feedstock can be converted to biocrude, leading to much higher biomass-to-biocrude yields. FIG. 1 illustrates various types of biomass feedstocks that may be used to create biocrude via the HTL process, including cultured algae, marine biomass, food waste, wood waste, and animal waste. Biomass feedstocks may also include municipal biosolid waste and grease waste. Biomass-to-biocrude conversion rates have been reported to be in the 40%-50% range, with some rates as high as 65%.
However, one of the main barriers preventing the widespread adoption of the HTL technology to produce renewable fuels has been the high energy requirement to drive the HTL reactor, which requires temperatures of around 400° C. and pressures of around 20 MPa. Traditionally, the HTL process requires a prohibitively large energy input that in turn renders the process with a negative EROI. Furthermore, current HTL processes are conducted in batch mode.